The present invention is related to a method and system for generating a low voltage reference that operates with a low power supply voltage. More particularly, the present invention is related to a single cell voltage reference that operates on low-voltage power supplies such as a single cell battery.
Band-gap voltage references are used as voltage references in electronic systems. The energy band-gap of Silicon is on the order of 1.2V, and is independent from temperature and power supply variations. Bipolar transistors have a negative temperature drift with respect to their base-emitter voltage (Vbe decreases as operating temperature increases on the order of xe2x88x922 mV/deg C.). However, the thermal voltage of a bipolar transistor has a positive temperature drift (Vt=kT/q, thus Vt increases as temperature increases). The positive temperature drift in the thermal voltage (Vt) may be arranged to compensate the negative temperature drift in the bipolar transistor""s base-emitter voltage. Band-gap reference circuits use the inherent characteristics of bipolar transistors to compensate for temperature effects and provide a stable operating voltage over various power supply and temperature ranges.
One example bandgap reference circuit includes two bipolar transistors that are arranged with a common base. Two resistors are series connected between the emitter of the first bipolar transistor and a common ground. The emitter of second bipolar transistor is connected to the common point between the two resistors. The two bipolar transistors are arranged to provide a ten-to-one (10:1) current density difference with respect to one another. The ten-to-one current density results in a 60 mV difference between the base-emitter voltages of two bipolar transistors (xcex94Vbe=Vt*ln(I1/I2)=26 mV*ln(10)=60 mV, at room temperature). The 60 mV difference appears across the first resistor. The voltage between the base of the bipolar transistors and the ground terminal provides a voltage reference (VREF) that is roughly given as VREF=Vbe+X*Vt, where X is a constant that is used to scale the temperature correction factor. The temperature correction factor (X) is adjusted by the ratio of the resistors. Typical temperature corrected reference voltages of 1.25V are achieved by this configuration.
The present invention is directed to providing a voltage reference that operates from a low power supply voltage. More specifically the present invention is directed to providing a voltage reference that is below 1.2 volts and operated from a low power-supply voltage such as in battery-operated applications.
Briefly stated, a single cell voltage reference operates under low power supply requirements to provide a configurable voltage reference. The single cell voltage reference includes a diode device that is biased as a voltage source. Two series connected resistive devices are connected in parallel with the diode device. The diode is biased with a current that is proportional to delta Vbe/R, such that the impedance of the diode tracks R. Another current source that is also proportional to delta Vbe/R is provided at the junction of the two resistors such that the voltage across one of the two resistors may be employed as a reference voltage that is less than 1.2V. The ratio of the resistors and scales the reference voltage level. Voltages that are below 1.2 V are provided that are temperature compensated similar to a band-gap reference. The diode voltage as driven by a current source determines the lower limit of the reference voltage. The reference voltage may be combined with a buffer or an operational amplifier such that a regulated supply can be provided that is below 1.2V. Metal masks may be arranged to permit reconfiguration of the voltage reference cell for use above 1.2V, or to change the regulation voltage without redesigning the voltage reference cell.
In accordance with a feature of the invention, an apparatus is directed to provide a reference voltage that operates from a low-voltage power supply. The apparatus includes a current source circuit that provides a current that is proportional to a change in base-emitter voltage and inversely proportional to a reference resistance value. A diode circuit is coupled to the current source, wherein the diode circuit has an associated diode voltage. A first resistive circuit that is series coupled to a second resistive circuit, the first resistive circuit and the second resistive circuit sharing a common node, the first and second resistive circuits are arranged in parallel with the diode device to provide a first voltage contribution at the common node that is associated with the current source that is lower than the associated diode voltage. Another current source circuit provides another current to the common node, wherein the another current is proportional to the current such that a second voltage contribution is provided at the common node, wherein the reference voltage is given as the sum of the first voltage contribution and the second voltage contribution.
In accordance with another feature of the invention, an apparatus is directed to providing a regulated voltage from an unregulated power supply that has a first and a second power supply terminal, where the first terminal is coupled to the regulated voltage through a resistor circuit. The apparatus includes a first current source circuit that provides a first current to a first node in response to a control signal. A second current source circuit provides a second current to a second node in response to the control signal. A control signal generator provides the control signal in response to a reference current that is proportional to a change in base-emitter voltage and inversely proportional to a resistance such that the first current and the second current are proportional to the reference current. A diode circuit is coupled between the first node and the second power supply terminal, the diode circuit having an associated diode voltage. A resistance circuit is coupled between the first node and the second node. Another resistance circuit is coupled between the second node and the second power supply terminal. A buffer circuit that has an input that is coupled to the second node and an output that provides the regulated voltage such that the regulated voltage is associated with a reference voltage at the second node.
In accordance with an aspect of the invention, an apparatus is directed to providing a regulated voltage from an unregulated power supply that has a first and a second power supply terminal, where the first terminal is coupled to the regulated voltage through a resistor circuit. The apparatus includes a means for providing a first current is arranged to provide the first current in response to a control signal. A means for providing a second current is arranged to provide the second current in response to the control signal. A means for providing a control signal is arranged to provide the control signal in response to a reference current such that the first current and the second current are proportional to the reference current, wherein the reference current is proportional to a change in base-emitter voltage and inversely proportional to a resistance. A diode means is arranged to provide a diode voltage that is responsive to the first current. A first resistance means is coupled between the diode means and a reference node. A second resistance means is coupled between the reference node and the second power supply terminal. A means for buffering is arranged to produce the regulated voltage from a reference voltage that is associated with the reference node.
In accordance with another aspect of the invention, a method is directed to providing a regulated voltage from an unregulated power supply that is coupled to the regulated voltage through a series circuit. The method includes generating a first current that is proportional to a reference current, wherein the reference current is proportional to a change in a base-emitter voltage and inversely proportional to a resistance value, generating a second current that is proportional to the reference current, biasing a diode circuit with at least one of the first current and the second current to provide a diode voltage, coupling a first resistance circuit between the diode and a reference node, coupling a second resistance circuit to between the reference node and a circuit ground potential such that the first and second resistance circuits produce a portion of a reference voltage at the reference node by dividing the diode voltage, coupling the second current to the reference node to produce a portion of a reference voltage at the reference node such that associated temperature coefficients of the first resistance circuit and the second resistance circuit are cancelled by another associated temperature coefficient of the resistance value wherein the reference voltage is given by the sum of a first constant times the diode voltage and a second constant times the change in the base-emitter voltage, and buffering the reference voltage to produce the regulated voltage from the reference voltage.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently preferred embodiments of the invention, and to the appended claims.